Passive mixer with reduced second order intermodulation

ABSTRACT

The present disclosure generally relates to the field of receiver structures in radio communication systems and more specifically to passive mixers in the receiver structure and to a technique for converting a first signal having a first frequency into a second signal having a second frequency by using a third signal having a third frequency. A passive mixer for converting a first signal having a first frequency into a second signal having a second frequency by using a third signal having a third frequency comprises a cancellation component for generating a first cancellation signal for cancelling second order intermodulation components by superimposing the first signal weighted by a cancellation value on the third signal; and a mixing component having a first terminal for receiving the first signal, a second terminal for outputting the second signal, and a third terminal for receiving the first cancellation signal, wherein the mixing component is adapted to provide the second signal as output at the second terminal by mixing the first signal provided as input at the first terminal and the first cancellation signal provided as input at the third terminal.

RELATED APPLICATIONS

This application is a divisional application of pending U.S. Ser. No.15/816,349, filed Nov. 17, 2017, which is a continuation application ofU.S. Ser. No. 13/503,168, filed Jun. 13, 2012, which issued as U.S. Pat.No. 9,825,590 on Nov. 21, 2017, which is the National Stage ofInternational Patent Application PCT/EP2009/007609, filed Oct. 23, 2009,the disclosures of which are incorporated herein by reference in theirentireties.

TECHNICAL FIELD

The invention generally relates to the field of receiver structures inradio communication systems. More specifically, the invention relates topassive mixers in the receiver structures and to a technique forconverting a first signal having a first frequency into a second signalhaving a second frequency by using a third signal having a thirdfrequency.

BACKGROUND

In radio communication systems, a mixer is used to up-convert a baseband(BB) signal or an intermediate frequency (IF) signal to a higherfrequency, e.g. a radio frequency (RF) signal, for ease of transmission(when the mixer is used in a transmitter) or to down-convert a highfrequency signal, e.g. an RF signal, to a lower frequency signal, e.g. aBB signal or an IF signal, for ease of signal processing (when the mixeris used in a receiver). The up-conversion or down-conversion isrespectively performed by mixing the input signal of the mixer with alocal oscillator (LO) signal generated by a local oscillator. In thereceiver case, the RF signal is mixed with the LO signal in order togenerate an IF signal or a BB signal.

In some mobile radio communication devices, the transmitter and receiverarchitectures are separated, i.e. separate circuitries are used for thereceiver and the transmitter. However, in other mobile communicationdevices, a transceiver is used, which is a device that has both atransmitter and a receiver, which are combined and share commoncircuitry. Transceivers normally include a duplexer which is a devicethat allows simultaneous bi-directional (full duplex) communication overa single channel. In radio communication systems, the duplexer isolatesthe receiver from the transmitter while permitting them to share acommon antenna.

A challenge in modern radio communication systems has been, andcontinues to be, to design receivers (and transmitters) that can meetincreasingly strict performance standards while fitting into evershrinking packages. To this end, many modern radio receivers (andtransmitters) are implemented on a single application specificintegrated circuit (ASIC). One of these strict performance standards isthe intermodulation requirement, in particular the so called secondorder intermodulation (IM2) requirement. Intermodulation can only occurin nonlinear systems. Nonlinear systems are generally composed of activecomponents, meaning that the components must be biased with an externalpower source which is not the input signal (i.e. the active componentsmust be “turned on”). However, even passive components can perform in anon-linear manner and cause intermodulation. Diodes or transistors arewidely known for their passive nonlinear effects, but parasiticnonlinearity can arise in other components as well. For example, audiotransformers exhibit non-linear behavior near their saturation point,electrolytic capacitors can start to behave as rectifiers underlarge-signal conditions, and RF connectors and antennas can exhibitnonlinear characteristics.

In the receiver case, a passive mixer generates IF or BB signals thatresult from the sum and difference of the LO and RF signals combined inthe mixer. These sum and difference signals at the IF port are of equalamplitude, but generally only the difference signal is desired forprocessing and demodulation so the sum frequency (also known as theimage signal) must be removed, typically by means of IF bandpass or BBlowpass filtering.

In the nonlinear case, further higher order components (caused byharmonics), like IM2 components, typically occur at the mixer output.The Second Order Intercept Point (IP2) is a measure of linearity thatquantifies the second-order distortion generated by nonlinear systemsand devices. At low power levels, the fundamental output power rises ina one-to-one ratio (in terms of dB) of the input power, while thesecond-order output power rises in a two-to-one ratio. When the inputpower is high enough and the device reaches saturation, the output powerflattens out in both the first- and second-order cases. The second orderintercept point is the point at which the first- and second-order linesintersect, assuming that the power levels do not flatten off due tosaturation. In other words, the IP2 is the theoretical point on the RFinput vs. IF output curve where the desired input signal and secondorder products become equal in amplitude as the RF input is raised.

Zero- and low-IF receiver architectures dominate today's low-costwireless receiver market for Time Division Multiple Access (TDMA) andTime Division Duplex (TDD) systems. For Frequency Division MultipleAccess (FDMA) systems and Code Division Multiple Access (CDMA) systems,like Wideband Code Division Multiple Access (WCDMA) systems, the strictIM2 and IP2 requirements typically necessitate more complex receiversolutions.

In a TDMA or TDD system, the wireless transmitter and receiver are noton at the same time but only in different, non-overlapping, time slots.Thus, for these systems the strongest receiver (Rx) interference is dueto an external transmitter, picked up via the antenna of the TDMA or TDDsystem. In FDMA or in CDMA systems, like in a WCDMA system, thestrongest Rx interferer is typically the wireless transmitter (Tx)itself, via leakage through the duplex filter of the system. Since thetransmitter leakage at full power typically is >10 dB stronger than anyexternal interferer, this will mainly set the IM2 and IP2 requirements.

For example, a WCDMA transmitter at +25 dBm power will result in a −25dBm Rx signal when the dupJexer attenuation is 50 dB. If only −108 dBmstatic Rx interference (an interference that is present all the time) isacceptable, the receiver IP2 has to be >+44 dBm for the rectified Txspectrum to be below the −108 dBm limit. For, e.g., GSM (Global Systemfor Mobile Communications), the strongest interferer is 5 dB lower or−30 dBm, resulting in a 10 dB lower IP2 requirement for the samedistortion levels.

Up till now, the common remedy for the high transmitter leakage levelshas been to introduce a filter between the low noise amplifier (LNA) andthe mixer of the receiver, typically an active mixer for noise reasons.Because of the small relative frequency separation between the closestTx and Rx band edges, i.e. the duplex gap, this filter typically is aSurface Acoustic Wave (SAW) filter which cannot be integrated into thetransceiver ASIC, but has to be located on the printed circuit board(PCB) or module substrate, adding to the cost and complexity of thereceiver structure.

Recently, alternating current (AC) coupling between the LNA and themixer core has been employed as a means to enhance IP2 by blockinglow-frequency IM2 noise to enter the mixer core, thereby preventing anyleakage due to mixer imbalances.

A passive metal oxide semiconductor (MOS) mixer offers good performancein terms of noise and linearity, especially when it's BB or IF port isat a virtual ground. The virtual ground eliminates the modulation of themixer switches by the BB or IF signal which improves IP2. Due to theinherent nature of the MOS device, its switching threshold and channelconductance depends on the LO, RF and IF signals. These interdependenceswill generate cross products of these signals, including ones that causeIM2. By grounding the IF port, e.g. via a virtual ground, some of thesecross products can be reduced resulting in less IM2 and consequently ahigher IP2. Still the switching threshold will be modulated by the RFsignal, resulting in an IM2 contribution in addition to that of thenonlinear channel conductance.

Today's AC-coupled mixer solutions provide enough performance when theduplexer isolation is 50 dB or better. For newer band configurationswith smaller duplex gaps and thus less duplexer isolation this may notbe possible. Also for cost reasons it may be advantageous to relax theseduplexer requirements, e.g. by allowing duplexers with less duplexerisolation, by improving the mixer IP2.

SUMMARY

Accordingly, there is a need to provide an improved and more costefficient passive mixer having improved IP2 performance.

This need is satisfied, according to a first aspect, by a passive mixerfor converting a first signal having a first frequency into a secondsignal having a second frequency by using a third signal having a thirdfrequency. The passive mixer comprises a cancellation component forgenerating a first cancellation signal for cancelling second orderintermodulation components by superimposing the first signal weighted bya cancellation value on the third signal; and a mixing component havinga first terminal for receiving the first signal, a second terminal foroutputting the second signal, and a third terminal for receiving thefirst cancellation signal, wherein the mixing component is adapted toprovide the second signal as output at the second terminal by mixing thefirst signal provided as input at the first terminal and the firstcancellation signal provided as input at the third terminal.

According to a second aspect, the above need is satisfied by a furtherpassive mixer for converting a first signal having a first frequencyinto a second signal having a second frequency by using a third signalhaving a third frequency. The passive mixer comprises a cancellationcomponent for generating a second cancellation signal for cancellingsecond order intermodulation components by superimposing the firstsignal weighted by a cancellation value on a bias, or reference,voltage; and a mixing component having a first terminal for receivingthe first signal, a second terminal for outputting the second signal, athird terminal for receiving the third signal, and a fourth terminal forreceiving the second cancellation signal, wherein the mixing componentis adapted to provide the second signal as output at the second terminalby mixing the first signal provided as input at the first terminal andthe third signal provided as input at the third terminal together withthe second cancellation signal provided as input at the fourth terminal.

The cancellation component according to both aspects may be adapted toapply only one of the first and second cancellation signals or both thefirst cancellation signal and the second cancellation signal. It isconceivable that the mixing component may comprise four terminals, thefirst to fourth terminals according to the second aspect, but may onlyuse three of the four terminals, e.g. the first to third terminals. Forexample, the three terminals are used by providing them with therespective signals, the fourth terminal remains unused and the mixingcomponent then generates the first cancellation signal withoutconsidering the fourth signal (bias voltage). If the bias voltage wouldadditionally be provided, the mixing component may generate the secondcancellation signal using the four terminals. The mixing component maybe adapted to receive both the first and second cancellation signals andto provide the second signal as output at the second terminal by mixingthe first signal provided as input at the first terminal and the firstcancellation signal provided as input at the third terminal togetherwith the second cancellation signal provided as input at the fourthterminal.

In accordance with both aspects, the mixing component may comprise avoltage controlled switch. For example, the mixing component comprises afield effect transistor switch, like a Metal Oxide Semiconductor FieldEffect Transistor (MOSFET), a Junction Field Effect Transistor (JFET), aMetal Semiconductor Field Effect Transistor (MESFET) and the like or atransistor providing similar characteristics as a field effecttransistor, like an Insulated Gate Bipolar Transistor (IGBT). The fieldeffect transistor switch may have its drain operatively connected to thefirst terminal, its gate operatively connected to the third terminal andits source operatively connected to the second terminal. In particularaccording to the second aspect, the field effect transistor switch mayhave its drain operatively connected to the first terminal, its gateoperatively connected to the third terminal, its source operativelyconnected to the second terminal and its bulk connected to the fourthterminal.

As is well known, source and drain may be interchanged for a symmetricdevice and what constitutes the drain and the source of a field effecttransistor may be bias, or signal, dependent. For simplicity, butwithout loss of generality, we assume that the drain is connected to thefirst terminal and the source to the second terminal. The mixingcomponent may alternatively comprise more than one voltage controlledswitch, for example two complementary voltage controlled switches. Inthis case, one N-channel transistor and one P-channel transistor may beconnected in parallel to each other, and may share the first and secondterminals, but may have distinct third terminals and, in case of thesecond aspect, distinct fourth terminals, e.g. distinct gate and bulkterminals.

The passive mixer may be used in a receiver arrangement for example of atransceiver comprising both a transmitter and a receiver. In this case,the passive mixer may be used in at least one of the receiver and thetransmitter of the transceiver. For example, the passive mixer isrealized as an integrated circuit provided on the same applicationspecific integrated circuit (ASIC) as the transceiver.

According to a first variant, the passive mixer may be arranged in thereceiver. The first signal may then be a radio frequency (RF) signalreceived by the receiver, the third signal may be a local oscillator(LO) signal provided by a local oscillator arranged in the receiver, andthe second signal may be one of an intermediate frequency (IF) signaland a baseband (BB) signal dependent on whether the passive mixer isadapted to perform direct conversion or indirect conversion. When thepassive mixer is adapted to perform indirect conversion, the received RFsignal is down-converted by the passive mixer into an IF signal having acenter frequency different from zero. The IF signal may then be furtherdown-converted by a similar or a different mixer into a BB signal. Incase of direct conversion, the RF signal is directly converted into a BBsignal having a center frequency equal to zero, i.e. having a spectrumof a certain bandwidth around zero.

According to a second variant, which can be combined with the firstvariant, the passive mixer may be arranged in the transmitter. The firstsignal may then be one of an IF signal and a BB signal dependent onwhether the passive mixer is adapted to perform direct conversion orindirect conversion, the third signal may be an LO signal provided by anLO arranged in the transmitter, and the second signal may be an RFsignal to be transmitted by the transmitter.

The second order intermodulation (IM2) components to be cancelled maycomprise terms dependent on the voltage at the first terminal of themixing component (the input voltage of the mixing component). Accordingto one variant, the second terminal (output terminal) of the mixingcomponent connected to a virtual ground or may have a voltage close toground such that the voltage swing at this terminal is very low comparedto the voltage swing of the first terminal. The IM2 components may thenbe proportional to the square of the voltage at the first terminal.Alternatively, in accordance with another variant, the voltage swing atthe second terminal may not be much less than that of the first terminaland the I 2 components may be dependent on both the voltage at the firstterminal and the voltage at the second terminal. For example, the IM2components are dependent on the voltage difference between the voltageat the first terminal and the voltage at the second terminal.

In order to cancel the IM2 components, both the cancellation values forthe first cancellation signal and the second cancellation signal may beset as fixed values. For example, both cancellation values may be equalto 0.5 or a value close to 0.5. Alternatively, the cancellation valuesmay initially be set to the fixed value and may then be adapted when theoperating conditions of the passive mixer change, e.g. when thetemperature of the passive mixer changes during operation. In yetanother example, one cancellation value may be fixed and the other maybe adapted to a value that depends on one or more of the operatingconditions, device mismatches, process spread or temperature.

As outlined above, the mixing component may comprise a field effecttransistor switch having its drain operatively connected to the firstterminal, its gate operatively connected to the third terminal and itssource operatively connected to the second terminal. In accordancetherewith, the IM2 components to be cancelled may comprise a termproportional to the second order of the drain source voltage.

According to a first realization of both the first and second aspect,the passive mixer may be used in current mode, i.e. the second terminal(output terminal) of the mixing component may be grounded or may be atvirtual ground. If the mixing component in this realization comprises afield effect transistor switch, the drain of the field effect transistormay receive the RF signal and the source of the field effect transistormay be connected to ground or to virtual ground. In order to determinethe quantity of the IM2 components, which may be proportional to thesquare of the drain source voltage of the field effect transistor, afirst sensing component may be used for sensing the voltage at the firstterminal, e.g. the voltage at the drain of the field effect transistor.In the current mode (the first realization) the second terminal (outputterminal) of the passive mixer is connected to ground or to virtualground, so that, as outlined above, the IM2 components may beproportional to the square of the voltage at the first terminal, i.e.the square of the voltage of the RF signal. In order to determine thecancellation value for weighting the first signal, e.g. the F signal, inaccordance with the first realization, the voltage sensed at the firstterminal by the first sensing component may be considered. According tothe first aspect, the weighted first signal, e.g. the weighted RFsignal, may then be superimposed on the third signal, e.g. the localoscillator signal, in order to generate the first cancellation signal.Alternatively, in accordance with the second aspect, the weighted firstsignal, e.g. the weighted RF signal, may be superimposed on the biasvoltage, e.g. the bulk voltage of the field effect transistor, in orderto generate the second cancellation signal. In line with the firstrealization of the first aspect, the first sensing component may beconnected to the first terminal and to the cancellation component, inorder so sense the voltage at the first terminal and in order to providethe weighted first signal to the cancellation component. Likewise, thecancellation component may be connected to the first terminal via thefirst sensing component and to the third terminal in order to receivethe weighted first signal and to superimpose the weighted first signaland the third signal. In accordance with the first realization of thesecond aspect, the first sensing component may be connected to the firstterminal and to the cancellation component, in order to sense thevoltage at the first terminal and in order to provide the weighted firstsignal to the cancellation component. Likewise, the cancellationcomponent may be connected to the first terminal via the first sensingcomponent and to the fourth terminal in order to receive the weightedfirst signal and to superimpose the weighted first signal and the biasvoltage.

Alternatively, in a second realization in particular of the firstaspect, the second terminal of the mixing component may not be connectedto a virtual ground and the passive mixer may not be used in currentmode, but in voltage mode. In the voltage mode, the second terminal ofthe mixing component, e.g. the source of the field effect transistor, isnot connected to a virtual-ground node, but may be connected to acapacitor which itself may be connected to ground. In this way, thesecond terminal of the mixing component may be loaded by the capacitorthat may provide an RF short to ground. The RF signal at the firstterminal and the IF signal or BB signal at the second terminal may bewidely separated in frequency. Therefore, the voltages at the firstterminal and the second terminal may be sensed independently from eachother.

For example, the passive mixer comprises, in addition to the firstsensing component for sensing the voltage at the first terminal, asecond sensing component for sensing the voltage at the second terminal.The second sensing component may be connected to the second terminal inorder to sense the voltage at the second terminal and to thecancellation component in order provide the cancellation component withthe information about the sensed voltage at the second terminal. Thecancellation component may not only be connected to the first sensingcomponent and to the third terminal but may further be connected to thesecond terminal via the second sensing component in order to generatethe first cancellation signal by not only considering the sensed voltageat the first terminal but by additionally considering the sensed voltageat the second terminal.

In accordance with a further variant, the passive mixer may furthercomprise two or more mixing components and a more-phase generator forgenerating the third signal with two or more different phases. Accordingto this variant, the more-phase generator may be supplied by twoopposing signal sources and may in this way float with respect to groundin order to generate two or more phases of the third signal. The twoopposing current sources for supplying signal with opposing phases mayby locally decoupled via a capacitor. The different phases of the thirdsignal may be individually fed into one or more of the two or moremixing components. For example, both of the two or more different phasesmay be fed into all of the two or more mixing components. Alternatively,one of the different phases may be supplied to one of the mixingcomponents, another of the different phases may be supplied to anotherone of the mixing components and so on.

For example, in accordance with this further variant, the first signalweighted by the cancellation value is superimposed on one phase of thethird signal and the first signal weighted by the same or an adaptedcancellation value is superimposed on another phase of the third signalin order to generate the first cancellation signal having multiplephases. In case of several different phases of the third signal, eachphase of the third signal may be modulated with the appropriate weightedfirst signal in order to generate the first cancellation signal at therespective mixing component.

In accordance with the second aspect, the cancellation component may beadapted to generate the second cancellation signal by superimposing thefirst signal weighted by the cancellation value on the bias voltage. Thedifferent phases of the third signal may then be provided to the thirdterminal of one or more of the two or more mixing components.

In accordance with both aspects, it may also be possible to turn off theweighted first signal, e.g. the weighted RF signal, at the cancellationcomponent in order to save power (e.g. by turning off the first senseamplifier), when the transmission power and thus the interferenceintroduced into the receiver is lower than a certain threshold.Additionally to the IM2 components from the own transmitter, the passivemixer may be adapted to also consider IM2 due to other devices, e.g.base stations.

The above need is also satisfied, according to a third aspect, by atransceiver apparatus comprising a transmitter for transmitting a radiofrequency transmit signal and a receiver for receiving a radio frequencyreceive signal. The receiver of the transceiver apparatus comprises alow noise amplifier for amplifying the high frequency receive signal;and a passive mixer comprising a local oscillator for generating a localoscillator signal; a cancellation component for generating a firstcancellation signal for cancelling second order intermodulationcomponents by superimposing the amplified radio frequency receive signalweighted by a cancellation value on the local oscillator signal; and amixing component having a first terminal for receiving the amplifiedradio frequency receive signal, a second terminal for outputting one ofan intermediate frequency signal and a baseband signal, and a thirdterminal for receiving the first cancellation signal, wherein the mixingcomponent is adapted to provide one of the intermediate frequency signaland the baseband signal as output at the second terminal by mixing theamplified radio frequency receive signal provided as input at the firstterminal and the first cancellation signal provided as input at thethird terminal.

According to a fourth aspect, the above need is satisfied by a furthertransceiver apparatus comprising a transmitter for transmitting a radiofrequency transmit signal and a receiver for receiving a radio frequencyreceive signal. The receiver of the transceiver apparatus comprises alow noise amplifier for amplifying the high frequency receive signal;and a passive mixer comprising a local oscillator for generating a localoscillator signal; a cancellation component for generating a secondcancellation signal for cancelling second order intermodulationcomponents by superimposing the amplified radio frequency receive signalweighted by a cancellation value on a bias voltage; and a mixingcomponent having a first terminal for receiving the amplified radiofrequency receive signal, a second terminal for outputting one of anintermediate frequency signal and a baseband signal, a third terminalfor receiving the local oscillator signal, and a fourth terminal forreceiving the second cancellation signal, wherein the mixing componentis adapted to provide one of the intermediate frequency signal and thebaseband signal as output at the second terminal by mixing the amplifiedradio frequency receive signal provided as input at the first terminaland the local oscillator signal provided as input at the third terminaltogether with the second cancellation signal provided as input at thefourth terminal.

According to both, the third and the fourth aspect, the receiver mayfurther comprise one of a bandpass filter and a lowpass filter connectedto the second terminal. In case of direct conversion, i.e. when thepassive mixer is adapted to directly convert the radio frequency receivesignal into a baseband signal, a lowpass filter may typically beconnected to the second terminal of the mixing component for filteringthe baseband signal via a passband of a predetermined frequency range.In case of indirect conversion, i.e. when the passive mixer is adaptedto convert the radio frequency receive signal into an intermediatefrequency signal, a bandpass filter having a passband of a predeterminedfrequency range may typically be connected to the second terminal of themixing component for filtering the intermediate frequency signal outputat the second terminal. The lowpass filter may be augmented by ACcoupling (i.e. a highpass filter) when there is little signal energy atDC (e.g. for WCDMA) and the bandpass filter may be implemented as acombination of a high-pass (or AC coupling) and a lowpass filter.

According to a fifth aspect, a mobile communication terminal isprovided, the mobile communication terminal comprising the transceiverapparatus according to the third or fourth aspect as outlined above.

The above need is further satisfied, according to a sixth aspect, by amethod for converting a first signal having a first frequency into asecond signal having a second frequency by using a third signal having athird frequency. The method comprises the steps of generating, by acancellation component, a first cancellation signal for cancellingsecond order intermodulation components by superimposing the firstsignal weighted by a cancellation value on the third signal; receiving,at a first terminal of a mixing component, the first signal; receiving,at a third terminal of the mixing component, the first cancellationsignal; and outputting, at a second terminal of the mixing component,the second signal by mixing the first signal provided as input at thefirst terminal and the first cancellation signal provided as input atthe third terminal.

According to a seventh aspect, the above need is satisfied by a furthermethod for converting a first signal having a first frequency into asecond signal having a second frequency by using a third signal having athird frequency. The method comprises the steps of generating, by acancellation component, a second cancellation signal for cancellingsecond order intermodulation components by superimposing the firstsignal weighted by a cancellation value on a bias voltage; receiving, ata first terminal of a mixing component, the first signal; receiving, ata third terminal of the mixing component, the third signal; receiving,at a fourth terminal of the mixing component, the second cancellationsignal; and outputting, at a second terminal of the mixing component,the second signal by mixing the first signal provided as input at thefirst terminal and the third signal provided as input at the thirdterminal together with the second cancellation signal provided as inputat the fourth terminal.

The cancellation value with which the first signal is weighted in orderto generate the first and second cancellation signals may be a fixedvalue, e.g. 0.5 or a value around 0.5. Alternatively, other values ofthe fixed cancellation values may be used which take into account theoperating conditions, e.g. which provide a good compromise over processand temperature. It is also conceivable that a set of differentcancellation values may be provided and that one of the set of differentcancellation values may be selected based on the operating conditions.The set of fixed cancellation values may comprise different values fordifferent operating conditions.

The cancellation value may also be determined based on a voltage sensedby a first sensing component at the first terminal. In addition, avoltage at the second terminal may be sensed by a second sensingcomponent and the first cancellation signal, the second cancellationsignal or both the first and the second cancellation signals may begenerated by additionally considering the sensed voltage at the secondterminal.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will further be described with referenceto exemplary embodiments illustrated in the figures, in which:

FIG. 1 is a block diagram schematically illustrating a transceiverapparatus embodiment;

FIG. 2 is a schematic illustration of a receiver of the transceiverembodiment of FIG. 1;

FIG. 3 is a schematic illustration of a first passive mixer embodimentof the receiver shown in FIG. 2;

FIG. 4 is a schematic illustration of a second passive mixer embodimentof the receiver shown in FIG. 2;

FIG. 5 is a flow chart illustrating a first method embodiment;

FIG. 6 is a schematic illustration of a third passive mixer embodimentof the receiver shown in FIG. 2;

FIG. 7 is a flow chart illustrating a second method embodiment;

FIG. 8 is a schematic illustration of a fourth passive mixer embodimentof the receiver shown in FIG. 2;

FIG. 9 is a schematic illustration of a fifth passive mixer embodimentof the receiver shown in FIG. 2;

FIG. 10 is a schematic illustration of a sixth passive mixer embodimentof the receiver shown in FIG. 2;

FIG. 11 is a schematic illustration of a seventh passive mixerembodiment of the receiver shown in FIG. 2;

FIG. 12 is a schematic illustration of an eighth passive mixerembodiment of a receiver shown in FIG. 2;

FIG. 13 is a schematic illustration of a ninth passive mixer embodimentof a receiver shown in FIG. 2;

FIG. 14 is a schematic illustration of a current-mode passive mixerembodiment of a transmitter shown in FIG. 1; and

FIG. 15 is a schematic illustration of a voltage-mode passive mixerembodiment of a transmitter shown in FIG. 1.

DETAILED DESCRIPTION

FIG. 13 is a schematic illustration of a ninth passive mixer embodimentof a receiver shown in FIG. 2 In the following description, for purposesof explanation and not limitation, specific details are set forth, suchas specific circuitries including particular components, elements etc.,in order to provide a thorough understanding of the present invention.It will be apparent to one skilled in the art that the present inventionmay be practiced in other embodiments that depart from these specificdetails. For example, the skilled person will appreciate that thepresent invention, although explained below with respect to a MetalOxide Semiconductor (MOS) Field Effect Transistor (FET), may make use ofother transistors like a Junction Field Effect Transistor (J FET), aMetal Semiconductor Field Effect Transistor (MESFET), an Insulated GateBipolar Transistor (IGBT) or the like. For example, the invention maymake use of n-channel MOSFETs, p-channel MOSFETs, n-channel JFETs orp-channel JFETs.

Those skilled in the art will further appreciate that functionsexplained herein below may be implemented using individual hardwarecircuitry and/or using an application specific integrated circuit(ASIC). The ASIC may be built from Field-programmable gate arrays(FPGAs), programmable logic devices (PLDs), like complex programmablelogic devices (CPLDs), or any other standard parts known to thoseskilled in the art. It will also be appreciated that when the presentinvention is described as a method, this method may also be embodied onthe ASIC.

FIG. 1 shows a block diagram of a radio frequency (RF) transceiverapparatus 110 for use in a mobile communication device 100. Asschematically illustrated in FIG. 1, the mobile communication device 100comprising the RF transceiver apparatus 110 is adapted to transmit aradio frequency transmit signal 102 from an antenna 112 and is adaptedto receive a radio frequency receive signal 104 with the antenna 112.The mobile terminal comprises a duplexer 114 connected to the RFtransceiver apparatus 110 via an impedance matching network 116 and apower amplifier 118, so that the transceiver apparatus 110 can be usedfor both transmitting the radio frequency transmit signal by means of atransmitter 130 and receiving the radio frequency receive signal bymeans of a receiver 120. The impedance matching network 116 is connectedto the receiver 120 of the RF transceiver apparatus 110 and the poweramplifier 118 is connected to the transmitter 130 of the RF transceiverapparatus 110.

The transmitter 130 of the RF transceiver apparatus 110 comprises adriver amplifier 310, a mixer 330, a local oscillator (LO) 340 and abaseband (BB) filter 350. The receiver 120 of the RF transceiverapparatus 110 comprises a low noise amplifier (LNA) 210, a mixer 230, alocal oscillator (LO) 240 and a baseband (BB) filter 250. When the RFtransceiver apparatus 110 is used in transmit mode, a data signal ispassed to the BB filter 350, filtered by the BB filter 350 and passed tothe mixer 330, where the BB data signal is up-converted into an RFsignal using an LO signal generated by the LO 340. The RF signal is thenpassed to the driver amplifier 310, the power amplifier 118, theduplexer 114 and finally to the antenna 112 for transmitting the RFtransmit signal 102. When the transceiver apparatus 110 is used inreceive mode, an RF receive signal 104 is received by the antenna 112,is passed by the duplexer 114 to the impedance matching network 116 andthen to the receiver 120 of the RF transceiver apparatus 110. In thereceiver 120, the LNA 210 amplifies the RF receive signal, the mixer 230directly down-converts the amplified RF receive signal into a BB signalby mixing the amplified RF receive signal with an LO signal generated bythe LO 240 and then passes the BB signal to the BB filter 250 forfurther BB filtering and amplification.

FIG. 2 illustrates the receiver 120 of the RF transceiver apparatus 110shown in FIG. 1 comprising the LNA 210 for amplifying the RF receivesignal, the mixer 230 for down-converting the amplified RF receivesignal into the BB signal by using the LO signal generated by the LO 240and the BB amplifier 250 for filtering and amplifying the down-convertedBB signal.

FIG. 3 schematically illustrates a first passive mixer 230 embodiment ofthe receiver 120 shown in FIG. 2. As shown in FIG. 3, the passive mixer230 comprises a MOSFET 231 as a mixing component having a drain, a gateand a source (the bulk is grounded for ease of simplicity), the drainbeing operatively connected to a first terminal 232, the source beingoperatively connected to a second terminal 234 and the gate beingoperatively connected to a third terminal 236 of the passive mixer 230.The first terminal 232 is adapted to receive the RF receive signal(amplified by the LNA 210), the second terminal 234 is adapted to outputthe BB signal and the third terminal is adapted to receive a thirdsignal, a first cancellation signal, the generation of which will bedescribed in more detail below.

The current through an N-channel MOSFET in its linear region, i.e. when0<Vds<Vgs−Vth, can to a first order be given as

$\begin{matrix}{I_{ds} = {\beta \times V_{ds} \times \left( {V_{gs} - V_{th} - \frac{V_{ds}}{2}} \right)}} & (1)\end{matrix}$

where I_(ds) is the drain-source current, β is a geometry dependentconstant, V_(gs) is the gate-source voltage, V_(ds) is the drain-sourcevoltage and Vth is the MOSFET threshold voltage. Here, an N-channeldevice has been assumed but similar relations can easily be derived forP-channel devices. Assuming, without loss of generality, that the sourceand the second terminal 234 are grounded, there will be two scenariosdepending on the polarity of V_(ds), namely the first scenario forV_(ds)>0 and the second scenario for V_(ds)<0. For V_(ds)>0, the secondterminal 234 will act as the source, i.e. the voltage at the sourceV_(s) will be equal to zero (grounded) (V_(s)=0), and the first terminal232 will act as the drain, i.e. the voltage at the drain V_(d) will beequal to the voltage of the RF signal V_(rf) (V_(d)=V_(rf)). ForV_(ds)<0, the drain and source are swapped (V_(d)=0 and V_(s)=V_(rf)).

For the first scenario (Vds>0), the drain-source current Ids becomes bymeans of equation (1)

$\begin{matrix}{I_{ds} = {\beta \times V_{rf} \times \left( {V_{lo} - V_{th} - \frac{V_{rf}}{2}} \right)}} & (2)\end{matrix}$

and for the second scenario (V_(ds)<0), the drain-source current Idsbecomes by means of equation (1)

$\begin{matrix}{I_{ds} = {{{- \beta} \times V_{rf} \times \left( {V_{lo} - V_{rf} - V_{th} + \frac{V_{rf}}{2}} \right)} = {{- \beta} \times V_{rf} \times \left( {V_{lo} - V_{th} - \frac{V_{rf}}{2}} \right)}}} & (3)\end{matrix}$

where V_(lo) is the voltage of the LO signal which is equal to thegate-source voltage V_(gs) since the source is grounded. The signreversal in the drain-source current Ids reflects the change inreference direction due to the terminal swapping.

As can be seen from equations (2) and (3), in both scenarios, there isone linear current component (V_(rf)×V_(lo)) and one second ordermodulation (IM2) component (V_(rf)×V_(rf)/2). Since the MOSFF switchprimarily works in the linear region, the above equations (2) and (3)describe the main influence of the nonlinear channel conductance on themixer current.

By superimposing a fraction of the RF signal on the LO signal byweighting the RF signal with a cancellation value α, i.e. when the gatevoltage Vg becomes

V _(g) =V _(lo) +α×V _(rf)  (4)

the cancellation of the IM2 term can be achieved by choosing thecancellation value α appropriately. Since the source is grounded V_(g)will equal V_(gs). As shown above, the IM2 component is proportional toV_(rf)*V_(rf)/2.

Thus, by selecting α=½ the IM2 component can be cancelled as equation(1) then yields in combination with equation (4)

$\mspace{79mu} {I_{ds} = {\beta \times V_{ds} \times \left( {V_{gs} - V_{th} - \frac{V_{ds}}{2}} \right)}}$${(1) + (4)} = {{\beta \times V_{rf} \times \left( {V_{lo} + \frac{V_{rf}}{2} - V_{th} - \frac{V_{rf}}{2}} \right)} = {\beta \times V_{rf} \times \left( {V_{lo} - V_{th}} \right)}}$

which is now proportional to V_(rf), i.e. is now linear, when V_(lo) andV_(th) can be considered constant.

Thus, by merely setting the cancellation value α=½, by weighting the RFsignal with the cancellation value α and by superimposing (adding) theweighted RF signal on (to) the LO signal, the IM2 component can becanceled.

The latter is exemplarily shown in FIG. 3, where the RF signal isweighted by the amplifier 222 with the cancellation value α (by scalingthe drive strength of the first sense amplifier 222 in relation to theLO) and is then added to the LO signal in order to generate the firstcancellation signal at the cancellation component 220. The firstcancellation signal is then provided to the third terminal 236(connected to the gate), the RF signal is provided to the first terminal232 (connected to the drain) and the BB signal is generated as output atthe second terminal 234 (connected to the source) by mixing the RFsignal and the first cancellation signal.

The above will cancel the IM2 due to the MOSFET 231 switch channelconductance, which covers most of the switch conduction angle. At theswitching threshold, the MOSFET 231 will start in the sub-thresholdregion and will enter the saturation region as soon as any significantcurrent starts to flow through the MOSFET 231. The drain-source currentI_(ds) in the saturation region can be described as

$\begin{matrix}{I_{ds} = {\frac{\beta}{2} \times \left( {V_{gs} - V_{th}} \right)^{2}}} & (5)\end{matrix}$

For V_(ds)>0, equation (5) yields

$\begin{matrix}{I_{ds} = {\frac{\beta}{2} \times \left( {V_{lo} - V_{th}} \right)^{2}}} & (6)\end{matrix}$

which is proportional to the square of V_(lo)and for V_(ds)<0, equation (5) yields

$\begin{matrix}{I_{ds} = {\frac{\beta}{2} \times \left( {V_{lo} - V_{rf} - V_{th}} \right)^{2}}} & (7)\end{matrix}$

which also has an IM2 term proportional to the square of V_(rf).

In the sub-threshold region, the drain-source current I_(ds) is muchsmaller and the characteristic is exponential also contributing withsome IM2.

When the cancellation value α is selected to deviate slightly from thelinear cancellation criterion, i.e. the cancellation value α would beselected to not equal 0.5, the IM2 generated in the sub-threshold regionand the saturation region can be compensated by allowing some residualIM2 in the linear region. In other words, the cancellation value α canbe tuned such that it nulls the sum of all IM2 contributions but doesnot null all individual IM2 components separately, e.g. the one in thelinear region.

As shown in FIG. 3, the cancellation component 220 is adapted togenerate the first cancellation signal for cancelling IM2 components bysuperimposing the RF receive signal weighted by the cancellation value αon the LO signal. Alternatively to setting the cancellation value α to afixed value, the first sense amplifier 222 can be used in order to sensethe voltage at the first terminal 232. By sensing the voltage at thefirst terminal 232 the appropriate cancellation value for cancelling theIM2 component can be determined by evaluating equation (1). Theunweighted RF receive signal is provided to the first terminal 232 ofthe MOSFET 231 and the first cancellation signal is provided to thethird terminal 236 of the MOSFET 231. By mixing the amplified RF receivesignal with the first cancellation signal, the MOSFET 231 switch outputsa BB signal at its source and thus at the second terminal 234 of themixing component. The BB signal is then filtered and amplified by the BBamplifier 250 comprising, for example as shown in FIG. 3, an amplifier252, a resistor 254 and a capacitor 256, wherein the resistor 254 andthe capacitor are operatively connected to the input and the output ofthe amplifier 252 for feedback control.

FIG. 4 shows a second mixer embodiment of the receiver shown in FIG. 2,which is used in voltage mode. In voltage mode, the source of the MOSFET231 and thus the second terminal 234 are not directly connected tovirtual ground but are connected via an impedance, e.g. as shown in FIG.4 a capacitor 258, to ground, i.e. the MOSFET 231 is loaded by thecapacitor 258. In contrast to the first passive mixer embodiment in FIG.3, the voltage at the second terminal 234 is not close to zero since thesecond terminal 234 is not connected to virtual ground. Therefore, thevoltage at the second terminal 234 has to be considered in equations (1)to (4) in order to determine the drain-source voltage V_(ds) and thegate-source voltage V_(gs). The drain-source voltage V_(ds) is unlike inthe first embodiment not merely equal to the RF voltage V_(rf) but isequal to the difference between the RF voltage V_(rf) at the firstterminal 232 and the IF voltage V_(s) at the second terminal 234.Likewise, the gate-source voltage V_(gs) (without any additionalcancellation signal) is not merely equal to the voltage of the LO signalV_(lo) but is equal to the difference between the voltage of the LOsignal and the source voltage V_(s) (the voltage at the second terminal234). Thus, in order to select the cancellation value α, both the drainvoltage V_(d) at the first terminal 232 and the source voltage V_(s) atthe second terminal 234 have to be sensed in order to determine thedrain-source voltage V_(ds) and the gate-source voltage V_(gs) of theMOSFET 231.

Since the RF voltage at the first terminal 232 and the IF voltage at thesecond terminal 234 are widely separated in frequency, they can besensed independently. In order to sense the voltage at the firstterminal 232, the first sense amplifier 222 is used and in order tosense the voltage at the second terminal 234, a second sense amplifier224 is used. Then, the sensed voltage at the first terminal 232 and thesensed voltage at the second terminal 234 are used to adapt thecancellation value α. The RF signal is weighted by the cancellationvalue α₁ and the IF signal is weighted by the cancellation value α₂ andthe weighted signals are provided to the cancellation component 220. Thecancellation values α₁ and α₂ may be the same for simplicity orindividually set to maximize performance. At the cancellation component220, the LO signal is superimposed on both the weighted RF signal andthe weighted IF signal, in order to generate a first cancellation signalwhich is then provided to the third terminal 236 and the gate of theMOSFET 231. The output of the second terminal 234 is then again providedto the BB amplifier 252 for amplification and filtering.

The two passive mixer embodiments described above with respect to FIGS.3 and 4 are further illustrated by the flow chart of FIG. 5 which showsa first method embodiment 300. The method 300 comprises the followingsteps: Generate first cancellation signal (step 302); Receive firstsignal at first terminal (step 304); Receive first cancellation signalat third terminal (step 306); and Output second signal at secondterminal by mixing first signal and first cancellation signal (step308).

FIG. 6 shows a third passive mixer embodiment for cancelling secondorder intermodulation components. The passive mixer is operated incurrent mode, like in the first passive mixer embodiment of FIG. 3, butin contrast to the first passive mixer embodiment shown in FIG. 3 (andalso to the second passive mixer embodiment shown in FIG. 4), the bulkterminal of the MOSFET 231, which is operatively connected to a fourthterminal 238 of the passive mixer, is additionally considered. Whenadditionally considering the bulk terminal 238, the threshold voltageV_(th) is proportional to the bulk-source voltage V_(bs)

V _(th) =V _(th0) −γ×V _(bs)  (8)

where V_(th) is the unmodulated threshold voltage and V_(bs) is thebulk-source voltage. By injecting (superimposing) a suitably scaledsecond cancellation signal at the bulk terminal 238, unwanted IM2components can also be suppressed. When V_(bs)=α×V_(rf). α×γ=½, andV_(g)=V_(lo) (since the source and the second terminal 234 aregrounded), the drain-source current Ids yields using equation (1)

$\begin{matrix}{I_{ds} = {{\beta \times V_{rf} \times \left( {V_{lo} - \left( {V_{{th}\; 0} - {\gamma \times \alpha \times V_{rf}}} \right) - \frac{V_{rf}}{2}} \right)} = {\beta \times V_{rf} \times \left( {V_{lo} - V_{{th}\; 0}} \right)}}} & (9)\end{matrix}$

Thus, the IM2 term is again cancelled with the above assumptions.

Since, in practice, V_(th) is a complex nonlinear function of the bulk,source and drain voltages, the above linearized model is thus not exact,but provides a good estimation. Also because of the small moderate andweak inversion conduction angles, the cancellation criterion α×γ can beselected to slightly deviate from 0.5 in order to minimize the aggregateIM2 components.

As shown in FIG. 6, the amplified RF receive signal is provided to thedrain of the MOSFET 231 and the first sense amplifier 242 is adapted tosense the voltage of the RF signal at the first terminal 232 in order tothe select the appropriate cancellation value. The RF receive signal isweighted with the determined cancellation value and is superimposed on abias voltage at the cancellation component 240 in order to generate thesecond cancellation signal. The second cancellation signal is providedto the bulk terminal of the MOSFET 231. The MOSFET 231 switch is adaptedto mix the amplified RF receive signal provided as input at the drainand the local oscillator signal provided as input at the gate togetherwith the second cancellation signal provided as input at the bulk of theMOSFET 231 in order to generate and output a BB signal at the source.The BB signal is then amplified by the BB amplifier 250.

The third passive mixer embodiment described above with respect to FIG.6 is further illustrated by the flow chart of FIG. 7 which shows asecond method embodiment 400. The method 400 comprises the followingsteps: Generate second cancellation signal (step 402); Receive firstsignal at first terminal (step 404); Receive third signal at thirdterminal (step 408); Receive second cancellation signal at fourthterminal (step 410); and Output second signal at second terminal bymixing first signal and third signal together with second cancellationsignal (step 412).

The first passive mixer embodiment of FIG. 3 and the third passive mixerembodiment of FIG. 6 can be combined to a fourth passive mixerembodiment as shown in FIG. 8. In this embodiment, the voltage at thefirst terminal 232 is sensed by two distinct sense amplifiers 222, 242,a first one 222 connected to the first terminal 232 and the thirdterminal 236 (via a first cancellation component 220) and a second one242 connected to the first terminal 232 and the fourth terminal 238 (viaa second cancellation component 240). In this way two distinctcancellation values can be determined by the two sense amplifiers 222,242, i.e. a first cancellation value determined by the first senseamplifier 222 and a second cancellation value determined by the secondsense amplifier 242. Then, a first cancellation signal is determined byadding the RF signal weighted by the first cancellation value to the LOsignal and a second cancellation signal is determined by adding the RFsignal weighted by the second cancellation value on a bias voltage. Asshown in FIG. 8, the first cancellation signal is input to the gate ofthe MOSFET 231 via the third terminal 236 and the second cancellationsignal is input to the bulk of the MOSFET 231 via the fourth terminal238. By mixing the first signal provided as input at the drain and thefirst cancellation signal provided as input at the gate together withthe second cancellation signal provided as input at the bulk, a BBsignal is provided at the source of the MOSFET 231 and the secondterminal 234. The BB signal is finally filtered and amplified by a BBamplifier 250.

In the fifth passive mixer embodiment of FIG. 9, like in the fourthembodiment of FIG. 8, also an additional sense amplifier 244 is added tothe third passive mixer embodiment of FIG. 6. Unlike the fourthembodiment shown in FIG. 8, the additional (second) sense amplifier 244is connected between the second terminal 234 and the second cancellationcomponent 240 rather than to the first terminal 232 and the firstcancellation component 220. In this way, the fifth passive mixerembodiment shown in FIG. 9 operates in voltage mode. The RF receivesignal is weighted with a first cancellation value (determined by thefirst sense amplifier 242 by sensing the voltage at the first terminal232) and, in addition thereto, the voltage (BB voltage) at the secondterminal 234 is sensed by the second sense amplifier 244 so that the BBsignal is weighted by a second cancellation value (determined by thesecond sense amplifier 244 by sensing the voltage at the second terminal234). Both the RF signal weighted by the first cancellation value andthe IF signal (BB signal) weighted by the second cancellation value areadded to a bias voltage at the cancellation component 240 in order togenerate the second cancellation signal. The second cancellation signalis provided to the bulk terminal of the MOSFET 231. The MOSFET 231switch is adapted to mix the amplified RF receive signal provided asinput at the drain and the local oscillator signal provided as input atthe gate together with the second cancellation signal provided as inputat the bulk of the MOSFET 231 in order to generate and output a BBsignal at the source. The BB signal is then amplified by the BBamplifier 250.

In the sixth passive mixer embodiment shown in FIG. 10, a further senseamplifier 224 is added to the fifth passive mixer embodiment of FIG. 9.The second cancellation signal which is provided to the bulk of theMOSFET 231, is determined in the way as described with respect to FIG. 9above. In addition thereto, the further sense amplifier 224 is adaptedto sense the voltage at the second terminal (like the sense amplifier244). Although the sense amplifier 244 and the sense amplifier 224 areboth adapted to sense the voltage at the second terminal 234, the BBsignal can be weighted with different cancellation values by the twoamplifiers 224, 244. The BB signal weighted by one cancellation value issupplied to the second cancellation component 240 (to generate thesecond cancellation signal) and the BB signal weighted by the same or adifferent cancellation value is supplied to the first cancellationcomponent 220 to be added to the LO signal in order to generate thefirst cancellation signal. By mixing the amplified RF receive signalprovided as input at the drain and the first cancellation signalprovided as input at the gate together with the second cancellationsignal provided as input at the bulk of the MOSFET 231, a BB signal isgenerated and output at the source and finally filtered and amplified bya BB amplifier 250.

The seventh passive mixer embodiment shown in FIG. 11 is a combinationof the previously described first to sixth passive mixer embodiments. Inthis embodiment, a first cancellation signal is generated bysuperimposing the RF signal weighted by a cancellation value and the BBsignal weighted by a cancellation value on the LO signal at the firstcancellation component 220 and a second cancellation signal is generatedby superimposing the RF signal weighted by a cancellation value and theBB signal weighted by a cancellation value on the bulk voltage at thesecond cancellation component 240. The first and second cancellationsignals are then provided to the respective terminals of the MOSFET 231(the gate and bulk) in order to generate the BB signal at the source asdescribed above with respect to the first to sixth embodiments.

An eighth passive mixer embodiment is shown in FIG. 12. According tothis embodiment, a more-phase generator, exemplarily shown as a 4-phasegenerator 270 in FIG. 12, is provided. The 4-phase generator comprisestwo opposing signal sources in order to generate different phases of theLO signal received by the 4-phase generator 270 as input. The RF receivesignal (amplified by the LNA 210) is provided at the drain of fourMOSFETs 231 shown in FIG. 12. In the same manner as described above, thevoltage of the RF signal (the voltage at the drain of the four MOSFETs230) is sensed by the first sense amplifier 222 and the RF receivesignal is weighted with a cancellation value by the first senseamplifier 222. The weighted RF receive signal is then added to thedifferent phases generated by the 4-phase generator 270 at therespective cancellation components 220. The RF signals applied to therespective cancellation components 220 may be weighted with the same orwith different cancellation values. If required by the operatingconditions (mismatch, process speed, temperature and the like), the RFsignal can be weighted with a first cancellation value and can be added(at a first cancellation component 220) to the first phase of the LOsignal generated by the 4-phase generator 270 and the RF signal can beweighted by a second cancellation value (different from the firstcancellation value) and can be added (at a second cancellation component220) to the second phase of the LO signal generated by the 4-phasegenerator and so on. In this way the appropriate cancellation signalsare determined.

The RF signal is then mixed by the four MOSFETs 231 with the respectivecancellation signals in the same manner as described above to output BB(or IF) signals at the source of the MOSFETs 231 which are finallyfiltered by the BB amplifiers 250 in order to generate I and Qquadrature components.

The ninth passive mixer embodiment illustrated in FIG. 13 differs fromthe eighth passive mixer embodiment of FIG. 12 in that the weighted RFsignal is superimposed on the bias voltage of the MOSFETs 231 ratherthan on the different phases of the LO signal. The different phases ofthe LO signal are instead supplied to the gate of the MOSFETs 231. Ateach MOSFET 231 the RF signal provided as input at the drain is mixedwith the respective phase of the LO signal provided as input at the gatetogether with the cancellation signal provided as input at the bulk, inorder to generate the BB (or IF) signal.

Although described herein primarily in the context of a receivercircuit, the IM2-suppressing passive mixer of the present invention isnot limited to receiver implementations, but additionally finds utilityin reducing the IM2, and consequently raising the IP2, in transmittercircuits. For example, FIG. 14 shows one embodiment of a passive mixer330, deployed in the transmitter 130 shown in FIG. 1. As shown in FIG.3, the passive mixer 330 comprises a MOSFET 331 as a mixing componenthaving a drain, a gate and a source (the bulk is grounded for ease ofsimplicity), the drain being operatively connected to a first terminal332, the source being operatively connected to a second terminal 334 andthe gate being operatively connected to a third terminal 336 of thepassive mixer 330. The first terminal 332 is adapted to receive a BB orIF signal (amplified by a BB or IF amplifier 310), the second terminal334 is adapted to output the RF signal and the third terminal is adaptedto receive a third signal, which is a first cancellation signal.

In one embodiment, the first cancellation signal is generated by settinga cancellation value α=½, by weighting the BB or IF signal with thecancellation value α and by superimposing (adding) the weighted BB or IFsignal on (to) the LO signal. The first cancellation signal thus cancelsthe IM2 component, in a manner similar to that described with respect tothe receiver circuit of FIG. 3.

In FIG. 14, the BB or IF signal is weighted by the amplifier 322 withthe cancellation value α (by scaling the drive strength of the firstsense amplifier 322 in relation to the LO) and is then added to the LOsignal in order to generate the first cancellation signal at thecancellation component 320. The first cancellation signal is thenprovided to the third terminal 336 (connected to the gate), the BB or IFsignal is provided to the first terminal 332 (connected to the drain)and the RF signal is generated as output at the second terminal 334(connected to the source) by mixing the BB or IF signal and the firstcancellation signal.

As described above with respect to the passive mixer in the receivercircuit 120 of FIG. 3, when the cancellation value α is selected todeviate slightly from the linear cancellation criterion, i.e. thecancellation value α would be selected to not equal 0.5, the IM2generated in the sub-threshold region and the saturation region can becompensated by allowing some residual IM2 in the linear region. In otherwords, the cancellation value α can be tuned such that it nulls the sumof all IM2 contributions but does not null all individual IM2 componentsseparately, e.g. the one in the linear region.

As shown in FIG. 14, the cancellation component 320 is adapted togenerate the first cancellation signal for cancelling IM2 components bysuperimposing the BB or IF signal weighted by the cancellation value αon the LO signal. In one embodiment, alternatively to setting thecancellation value α to a fixed value, the first sense amplifier 322 canbe used in order to sense the voltage at the first terminal 332. Bysensing the voltage at the first terminal 332 the appropriatecancellation value for cancelling the IM2 component can be determined byevaluating equation (1). The unweighted BB or IF signal is provided tothe first terminal 332 of the MOSFET 331 and the first cancellationsignal is provided to the third terminal 336 of the MOSFET 331. Bymixing the amplified BB or IF signal with the first cancellation signal,the MOSFET 331 switch outputs an RF signal at its source and thus at thesecond terminal 334 of the mixing component. The RF signal is thenfiltered and amplified by the RF amplifier 350 comprising, for exampleas shown in FIG. 14, an amplifier 352 having a resistor 354 operativelyconnected to the input and the output of the amplifier 352 for feedbackcontrol.

FIG. 15 shows an embodiment of a passive mixer 330, deployed in thetransmitter 130 shown in FIG. 1, which is used in voltage mode. Involtage mode, the source of the MOSFET 331 and thus the second terminal334 are not directly connected to virtual ground, but rather areconnected via an impedance, e.g. as shown in FIG. 15, a capacitor 358,to ground. That is, the MOSFET 331 is loaded by the capacitor 358. Incontrast to the passive mixer embodiment in FIG. 14, the voltage at thesecond terminal 334 is not close to zero, since the second terminal 334is not connected to virtual ground. Therefore, the voltage at the secondterminal 334 has to be considered in equations (1) to (4) in order todetermine the drain-source voltage V_(ds) and the gate-source voltageV_(gs). The drain-source voltage V_(ds) is, unlike in the embodiment ofFIG. 14, not merely equal to the RF voltage V_(rf), but it is equal tothe difference between the BB or IF voltage V_(bb) or V_(if) at thefirst terminal 332 and the RF voltage V_(s) at the second terminal 334.Likewise, the gate-source voltage V_(gs) (without any additionalcancellation signal) is not merely equal to the voltage of the LO signalV_(lo) but is equal to the difference between the voltage of the LOsignal and the source voltage V_(s) (the voltage at the second terminal334). Thus, in order to select the cancellation value α, both the drainvoltage V_(d) at the first terminal 332 and the source voltage V_(s) atthe second terminal 334 have to be sensed in order to determine thedrain-source voltage V_(ds) and the gate-source voltage V_(gs) of theMOSFET 331.

Since the BB or IF voltage at the first terminal 332 and the RF voltageat the second terminal 334 are widely separated in frequency, they canbe sensed independently. In order to sense the voltage at the firstterminal 332, the first sense amplifier 322 is used and in order tosense the voltage at the second terminal 334, a second sense amplifier324 is used. Then, the sensed voltage at the first terminal 332 and thesensed voltage at the second terminal 334 are used to adapt thecancellation value α. The BB or IF signal is weighted by thecancellation value α₁ and the RF signal is weighted by the cancellationvalue α₂ and the weighted signals are provided to the cancellationcomponent 230. The cancellation values α₁ and α₂ may be the same forsimplicity or individually set to maximize performance. At thecancellation component 320, the LO signal is superimposed on both theweighted BB or IF signal and the weighted RF signal, in order togenerate a first cancellation signal which is then provided to the thirdterminal 336 and the gate of the MOSFET 331. The output of the secondterminal 334 is then again provided to the RF amplifier 352 foramplification and filtering.

Those of skill in the art will readily recognize that all embodiments ofthe passive mixer described herein in the context of a receiver (e.g.,as depicted in FIGS. 3-4, 6, 8-13) may similarly be advantageouslydeployed in a transmitter circuit to reduce IM2 and hence raise the IP2.

The technique described above results in noticeable IM2 improvements onthe order of 20 dB, in particular for the current mode mixer. Thissignificant improvement leads to decreased duplexer requirements andtherefore to lower costs, smaller size and smaller losses. For frequencybands where the duplexer Tx-Rx isolation is below the typically required50 dB, the described technique eliminates the need for a SAW interstagefilter between the LNA and the mixer, which also improves bandflexibility and reduces the costs and size of the receiver structuresand the devices.

What is claimed is:
 1. A passive mixer adapted to convert a first signalhaving a first frequency into a second signal having a second frequencyby using a third signal having a third frequency, comprising: a firstcancellation component adapted to generate a first cancellation signaloperative to substantially cancel second order intermodulationcomponents by adding the first signal weighted by a first cancellationvalue on the third signal; and a mixing component having a firstterminal adapted to receive the first signal, a second terminal adaptedto output the second signal, and a third terminal adapted to receive thefirst cancellation signal, wherein the mixing component is adapted toprovide the second signal as output at the second terminal by mixing thefirst signal provided as input at the first terminal and the firstcancellation signal provided as input at the third terminal; and a firstactive sensing component adapted to sense a voltage at the firstterminal and to adapt the first cancellation value based on the sensedvoltage at the first terminal.
 2. The passive mixer according to claim1, wherein the mixing component comprises a pair of two complementaryvoltage controlled switches, wherein the two complementary voltagecontrolled switches are connected in parallel, share the first terminal,and each have a distinct third terminal.
 3. The passive mixer accordingto claim 1, wherein the mixing component comprises a field effecttransistor switch having its drain operatively connected to the firstterminal, its gate operatively connected to the third terminal and itssource operatively connected to the second terminal.
 4. The passivemixer according to claim 1, wherein the mixing component comprises afield effect transistor switch having its source operatively connectedto the first terminal, its gate operatively connected to the thirdterminal and its drain operatively connected to the second terminal. 5.The passive mixer according to claim 1, further comprising: a secondcancellation component adapted to generate a second cancellation signaloperative to substantially cancel second order intermodulationcomponents by adding the first signal weighted by a second cancellationvalue on a bias voltage; and a second sensing component adapted to sensethe voltage at the first terminal and to adapt the second cancellationvalue based on the sensed voltage at the first terminal; and wherein themixing component further has a fourth terminal adapted to receive thesecond cancellation signal, and wherein the mixing component is adaptedto provide the second signal as output at the second terminal by mixingthe first signal provided as input at the first terminal, the firstcancellation signal provided as input at the third terminal, and thesecond cancellation signal provided as input at the fourth terminal. 6.The passive mixer according to claim 5, further comprising: a thirdsensing component adapted to sense the voltage at the second terminal,wherein the first cancellation component is adapted to generate thefirst cancellation signal by additionally considering the sensed voltageat the second terminal; and a fourth sensing component adapted to sensethe voltage at the second terminal, wherein the second cancellationcomponent is adapted to generate the second cancellation signal byadditionally considering the sensed voltage at the second terminal. 7.The passive mixer according to claim 6, further comprising a secondsensing component adapted to sense the voltage at the second terminal,wherein the first cancellation component is adapted to generate thefirst cancellation signal by additionally considering the sensed voltageat the second terminal.
 8. The passive mixer according to claim 1,further comprising two or more mixing components and a correspondingnumber of cancellation components, and a more-phase generator adapted togenerate the third signal with two or more different phases and toindividually feed the different phases of the third signal into one ormore of the two or more mixing components.
 9. The passive mixeraccording to claim 8, wherein the more-phase generator comprises afour-phase generator, and wherein the cancellation components may beweighted with the same or different cancellation values.
 10. The passivemixer according to claim 8, wherein the first cancellation component isadapted to generate the first cancellation signal by superimposing thefirst signal weighted by the first cancellation value on one phase ofthe third signal and by superimposing the first signal weighted by thesame or an adapted cancellation value on another phase of the thirdsignal.
 11. The passive mixer according to claim 1, wherein the firstsignal is a radio frequency signal, the third signal is a localoscillator signal, and the second signal is one of an intermediatefrequency signal and a baseband signal.
 12. The passive mixer accordingto claim 1, wherein the first signal is one of an intermediate frequencysignal and a baseband signal, the third signal is a local oscillatorsignal, and the second signal is a radio frequency signal.
 13. Atransceiver apparatus comprising a transmitter adapted to transmit aradio frequency transmit signal and a receiver adapted to receive aradio frequency receive signal, wherein the receiver comprises: a lownoise amplifier adapted to amplify the high frequency receive signal;and a passive mixer comprising: a local oscillator adapted to generate alocal oscillator signal; a cancellation component adapted to generate afirst cancellation signal to substantially cancel second orderintermodulation components by superimposing the amplified radiofrequency receive signal weighted by a cancellation value on the localoscillator signal; and a mixing component having a first terminaladapted to receive the amplified radio frequency receive signal, asecond terminal adapted to output one of an intermediate frequencysignal and a baseband signal, and a third terminal adapted to receivethe first cancellation signal, wherein the mixing component is adaptedto provide one of the intermediate frequency signal and the basebandsignal as output at the second terminal by mixing the amplified radiofrequency receive signal provided as input at the first terminal and thefirst cancellation signal provided as input at the third terminal. 14.The transceiver apparatus according to claim 13, wherein the receiverfurther comprises one of a bandpass filter and a lowpass filterconnected to the second terminal, wherein the bandpass filter has apassband of a predetermined frequency range adapted to filter theintermediate frequency signal and the lowpass filter has a passband of apredetermined frequency range adapted to filter the baseband signal. 15.A transceiver apparatus comprising a transmitter adapted to transmit aradio frequency transmit signal and a receiver adapted to receive aradio frequency receive signal, wherein the transmitter comprises: anamplifier adapted to amplify one of an intermediate frequency signal anda baseband signal to be transmitted; and a passive mixer comprising: alocal oscillator adapted to generate a local oscillator signal; acancellation component adapted to generate a first cancellation signalto substantially cancel second order intermodulation components bysuperimposing the amplified intermediate frequency signal or basebandsignal weighted by a cancellation value on the local oscillator signal;and a mixing component having a first terminal adapted to receive theamplified intermediate frequency signal or baseband signal, a secondterminal adapted to output a radio frequency transmit signal, and athird terminal adapted to receive the first cancellation signal, whereinthe mixing component is adapted to provide the radio frequency transmitsignal as output at the second terminal by mixing the amplifiedintermediate frequency signal or baseband signal provided as input atthe first terminal and the first cancellation signal provided as inputat the third terminal.
 16. A method, performed by a passive mixer, ofconverting a first signal having a first frequency into a second signalhaving a second frequency by using a third signal having a thirdfrequency, comprising: receiving, at a first terminal of a mixingcomponent, the first signal; receiving, at a third terminal of themixing component, the first cancellation signal; sensing, by a firstsensing component, a voltage at the first terminal; adapting acancellation value based on the sensed voltage at the first terminal;generating, by a cancellation component, a first cancellation signal tosubstantially cancel second order intermodulation components bysuperimposing the first signal weighted by the cancellation value on thethird signal; and outputting, at a second terminal of the mixingcomponent, the second signal by mixing the first signal provided asinput at the first terminal and the first cancellation signal providedas input at the third terminal.
 17. The method of claim 16, wherein thevoltage at the second terminal is sensed by a second sensing componentand the first cancellation signal is generated by additionallyconsidering the sensed voltage at the second terminal.